Electromagnetic waves in wide frequency ranges from microwave, terahertz, IR, to photonics have been used for applications to time-domain imaging (TDI). Time-domain imaging has potential to offer a solution in numerous applications ranging from medical diagnostics to security for package screening, color testing, and explosive detection. In spite of all these potential applications, time-domain imaging suffers for low sampling rate, which mainly is caused by the slow speed of its optical delay lines (ODL), which is a major component of TDI. In realistic time-domain imaging, an oscillatory optical time delay window of 100 ps at 100 Hz is required. Presently, such an ODL with this performance is not available. To arrange a time delay window of 100 ps, retro-refracting mirrors are used to oscillate with amplitude of 1.5 cm to generate 3 cm optical path difference (OPD) using an appropriate mechanical shaker at resonance mode. Even though this technique supplies the 100 ps required optical delay line, it suffers from a low scanning rate (less than 20 Hz).
In TDI, the 100 ps optical delay requirement exists when the system is used for deep depth spectroscopy and when the layers of sample in depth are under test. In some cases, the sample depth study is limited and much lower time delay of 100 ps is required, but the system speed is critical. In these situations, the shaker machine could be replaced by a piezoelectric driver, to improve scanning speed to a required level of 100 Hz. The piezoelectric system suffers from high voltage power supply requirement for operating piezoelectric circuits.
Therefore, the today's available ODL techniques, suffer from low sampling rate and make the imaging time sometimes to several minutes or even hours, or the systems are only applicable for TDI which requires a small time delay for operation. In studying numerous TDI applications, it is cleared that both system requirements of 100 ps time delay, and repetition rate of 100 Hz, are essential to perform simultaneously.